iMac - RAM Module Question

I have an iMac 1.25 GHz 20" which uses PC2700 DDR SDRAM modules. In addition to the factory-installed 256 Mb module, I have added a 512Mb module.

I would like to upgrade to the machine-rated 1GB of RAM maximum. Thus, I have few quick (& remedial) questions:

(1) Can the factory-installed 256 Mb module be replaced or is it permanently installed?

(2) If it is permanently installed, I will have to replace the user-installed 512Mb module. Since (I assume) that the next larger size of RAM would be 1GB, would this be a problem (since total installed RAM would be 1.25GB)?

(3) Which manufacturer(s) would you recommend for high-quality RAM modules?

So by that definition you can have 2 GB of ram in your mac. 1 GB from apple in it's factory installed slot and 1 GB you add yourself. I think. I'm not sure if that factory Ram can be removed. I'm pretty sure it can be though.

So hmmm I think even if 1.0 GB is the max memory you can have. Having 1.25 GB of ram in your machine shouldn't affect it at all. Cause only 1 GB of it will be accessed by the machine and the last 0.25 GB will be just forgoten about. Well I think that's how it works. Anyone want to confirm of deny this?

3. If you're in the US a store called Newegg has been suggested here many times. I don't live in the US so I'm just taking the words of the members here.